A magnetic recording medium, particularly video tape, is required to have a capability of reproducing high frequency records with a high output: an excellent surface smoothness for reducing space loss: a magnetic substance having an excellent dispersibility and a high filling density of magnetic substances for improving electromagnetic conversion characteristics: and physical characteristics satisfying the durability and life in repetitive use over a long period of time.
A recording density of a magnetic recording medium increases with a decrease in a magnetic particle size. The decreased particle size, however, causes deterioration of dispersibility of the particles, lowering of surface smoothness of a magnetic layer necessary for high-frequency recordings, output fall and bad wear resistance, which in turn cause problems of clogging video head in a VTR still mode and abrasion of a magnetic layer surface.
Therefore, binder for a magnetic layer is required to have an excellent durability and wear resistance. Various vinyl chloride-vinyl acetate binders have so far been proposed, but few binders satisfy the above requirements.
A method for improving dispersibility of a magnetic powder in a binder is proposed, in which a polar group is introduced in a binder resin used for a magnetic layer to make the binder well affinitive with the magnetic powder.
For example, the binders having a high dispersibility obtained by introducing a hydrophilic group therein to improve the affinity thereof with magnetic powder are disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP O.P.I.) Nos. 44227/1982, 108032/1983, 1215414/1985 and 92422/1982.
JP O.P.I. No. 44227/1982 discloses vinyl chloride-vinyl acetate copolymers having metal sulfonate group; JP O.P.I. No. 108032/1983 discloses sulfonic acid group-containing vinyl chloride copolymers: JP O.P.I. No. 121514/1985 discloses vinyl chloride copolymers obtained by polymerizing monomers having a metal sulfonate group: and JP O.P.I. No. 92422/1982 discloses polyurethane resins having a polar group.
Further, JP O.P.I. Nos. 8127/1984 and 104329/1986 propose various combinations of vinyl chloride-vinyl acetate copolymers and polyurethane resins having a metal sulfonate group.
The resins having a polar group used as a resin component of a binder enable to provide a magnetic recording medium having improved electromagnetic conversion characteristics, in which a ferromagnetic metal powder is well dispersed in a magnetic layer.
However, the above binders cannot necessarily give sufficient dispersion to such substances as a magnetic powder with a specific surface area having a BET value of not less than 45 m.sup.2 /g and carbon black having a poor dispersibility, and further can not provide satisfactory properties such as a Young's modulus and a resistance to a high temperature of 40 to 50.degree. C.
As described above, there have scarecely been found the binders capable of sufficiently dispersing therein even a ultrafine magnetic powder and a less dispersible powder, and therefore it has been difficult to provide a magnetic recording medium containing a ultrafine magnetic powder while securing the prescribed levels of a durability, magnetic characteristics and electromagnetic conversion characteristics.